Generally, quartz crystal microbalance (QCM) immunosensors for bacterial detection solely measure the resonant frequency, and the frequency shift is usually correlated to an elastic mass effect. The quartz crystal microbalance (QCM), as a simple yet powerful technique, has been widely employed in chemical and biological sensing. QCM can be designed as an immunosensor for directly detecting microorganisms without the need of labeled antibodies that are required in sandwich-type immunosensors. QCM immunosensors have been reported for rapid and specific detection of different bacteria. Most QCM immunosensors solely measure the resonant frequency (F0) using the standard oscillator technique, and the frequency change (ΔF) is usually explained by Sauerbrey equation, which states that the decrease in F0(−ΔF) is linearly proportional to the increase in surface mass loading of QCM (Sauerbrey, 1959).
However, the Sauerbrey equation is applicable only to a thin (˜1 μm) and elastic film coupled to the crystal surface, where the mass loading can be up to 0.05% of the crystal mass. The Sauerbrey equation does not apply to the case of cells attached to the QCM surface, largely due to the softness and relatively large size of the cells. In addition to the mass effect, the changes of surface viscoelasticity and other factors also contribute to the frequency change. Due to the additive nature of these effects, the mass effect cannot be differentiated from others when only F0 is tracked.
High-frequency impedance/admittance analysis can provide more detailed information about the surface/interface changes of QCM. A QCM can be represented by a Butterworth-Van Dyke (BVD) model, which is composed of a static capacitance (C0) in parallel with a motional branch containing a motional inductance (Lm), a motional capacitance (Cm), and a motional resistance (Rm) in series. These parameters are determined by physical properties of the quartz crystal, perturbing mass layer, and contacting liquid, and can be obtained with a high-frequency impedance analyzer by fitting the measured impedance/admittance data to the BVD model. A simpler way to provide insights into the viscoelastic properties of the bound surface mass is to simultaneously monitor F0 and Rm or F0 and the dissipation factor D using a quartz crystal analyzer that is less expensive than the impedance analyzer. This method has been applied to study the behavior of adherent cells in response to chemical, biological, or physical changes in the environment.
The impedance analysis has been used to characterize a QCM immunosensor for detecting Salmonella Typhimurium. A magnetic force was utilized to collect the complexes of Salmonella-magnetic beads on the crystal surface, and Rm was found the most effective and sensitive among the four circuit parameters, which offered a detection limit of about 103 cells/ml. The sensitivity of the QCM immunosensor in the absence of magnetic beads has not been investigated nor has the measurements of Rm and F0, therefore it is unclear how much the magnetic beads could affect the detection sensitivity or which of the F0and Rm measurements is superior in the presence or absence of the beads.